Schedule of Information Related to Stock Repurchase Programs (Parenthetical) (Detail) - shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Restricted shares forfeited by former employee	431,375
Restricted Stock Purchase Plan
Class of Stock [Line Items]
Restricted shares forfeited by former employee	10,791	2,500

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration